While the invention is subject to a wide range of applications, it is particularly suited to measure the three-dimensional contour of a surface. In particular, this invention relates to that aspect of ophthalmic diagnosis which is concerned with measurement of the three-dimensional contour of the anterior surface of the cornea. This measurement discloses abnormalities in the cornea which may have deleterious effects upon vision or quantifies progress of ophthalmic surgery, such as laser-aided radial keratotomy or laser ablation of the external surface of the cornea with penetration into the stroma and volumetric removal of tissue, whereby the external corneal surface is characterized by a sculptured, new curvature having improved optical properties.
Devices variously called corneascopes or keratometers have been developed for topographic analysis of the cornea. Such devices have found acceptance as means for measuring corneal curvature in preparation for prescribing a contact lens to be worn over the measured cornea to reduce certain visual defects, or for use in other ophthalmic applications. The prior art for these devices entails photographic (as in U.S. Pat. No. 3,797,921, Kilmer, et al.) or electro-optical (see U.S. Pat. No. 4,572,628, Nohda) recording of cornea-reflected images of illuminated objects comprising several concentric rings, or multiple discrete light sources arranged in the form of concentric rings, on a flat surface normal to the optical system axis or on a concave surface symmetrically disposed with respect to that axis. If the cornea is spherical, the reflected images of these ring-shaped objects are equally spaced, continuous or intermittent, concentric ring-shaped patterns. If the cornea surface is rotationally symmetrical but not spherical, the resultant ring images are less equally spaced; the inequality of spacing is thus a measure of nonsphericity of the cornea surface. If the cornea surface is astigmatic, as is frequently the case, the ring-shaped images reflected by that cornea will appear elliptical, and the eccentricity of the pattern is related to the change in curvature of the cornea surface between various sectional meridians. This eccentricity, and hence the astigmatism of the surface, can be measured by careful analysis of an image of the ring pattern. The orientation of the major and minor axes of the elliptical pattern relative to the eye indicates the orientation of the principal axes of the observed astigmatism. If the cornea has been warped or distorted by injury, by disease or by prior surgical procedures, such as radial keratotomy or imperfect closure of incisions made during cataract or other surgery, the magnitudes of these surface defects can also be measured.
In each of these described cases, the desired end result is (1) a tabular or graphic representation of the surface optical power (in units of diopters) at various points over the visually used, central portion of the cornea (typically 3 to 7 mm in diameter), and (2) computed average values for these parameters over the area of interest. Because of the tendency for the eye to become astigmatic, or non-rotationally symmetrical, comparisons of surface radius or power are frequently made for various azimuthal meridians about the visual axis. Instrument errors introduced by the apparatus and systematic or random errors introduced by the method of use are preferably minimized in order to minimize the overall measurement error. Prior art devices for accomplishing these measurements have been found lacking in regard to one or more of the following attributes: accuracy, ease of use, and time required to obtain the desired tabular or graphical output. None of these devices is compatible with use in situ and in close temporal alignment with surgical laser sculpturing of the cornea to produce desired net curvature changes to improve vision.